The present invention relates to a method and an apparatus for the defined regeneration of sooty surfaces, in particular of ceramic sensor surfaces.
In the course of environmental efforts to reduce the expulsion of soot from diesel engines, it becomes necessary to be able to determine the concentration of soot particles in the exhaust gas in a simple way. In particular, monitoring the soot content downstream of a diesel particle filter (DPF) during driving makes sense. For monitoring regeneration, moreover, the loading of a diesel particle filter must be predicted, for the sake of achieving high system security.
For determining the soot concentration in the exhaust gas of internal combustion engines, a sensor with a device for detecting soot particles can be mounted in the exhaust pipe.
From German Patent Disclosure DE 33 04 548 A1, a sensor is known that has electrodes on a nonconductive surface. The soot concentration can be derived from the conductivity, or the change in conductivity from soot deposition on the creep current surface located between the electrodes.
This measuring method is equivalent to a cumulative measurement principle, and the sooty sensor surface must therefore be freed of the conductive soot particles from time to time. For regeneration of the sooty surface, it is proposed that a high voltage be applied between the electrodes, in order to burn off the soot particles by way of the flow of current.
One disadvantage of the procedure proposed above arises from the fact that the electrodes themselves can be damaged by the thermal and electrical load, for instance from spark development between the electrodes, and the electromagnetic compatibility (EMC) of this method is a problem.
Moreover, especially with sensors with a ceramic multilayer construction, the integration of a heater and a temperature measuring element by thick-film technology suggests itself for regenerating the sensor surface. The regeneration is then done by simple heating to above the soot burnoff temperature. However, in this method as well, there is the disadvantage that because of the heating and cooling of the sensor and its thermal inertia, situations arise in which the soot deposition occurs in an undefined way and is difficult to monitor. In particular, during regeneration above 650° C., no soot is deposited; that is, during this phase, the sensor is incapable of making measurements.
As a result, it is impossible to draw a conclusion about a deposition rate in the regeneration phase, or the deposition rate on the heated sensor surface, regardless of the actual concentration of soot particles in the exhaust system, is equal to zero. At the same time, controlled adjustment of the rate of removal of the soot particles is made more difficult by the kind of method described above. Another possible way of freeing a sooty surface of soot is to burn off the soot particles by means of a dielectrically hindered discharge (DHD).
In a DHD, because a dielectric is positioned between the discharge electrodes, the development of a spark or an arc in the plasma upon application of a high voltage is suppressed, resulting in a nonthermal plasma. Typically, a DHD is excited with pulsating or alternating voltage. Gas discharges ignited in alternation in opposite directions break down the charge of the dielectric again before it can be built up significantly. Significant heating of the gas therefore does not occur.
If soot particles are conducted through a nonthermal plasma, radicals form and trip plasma-chemical reactions at the soot surfaces, and the soot particles become oxidized. Using the DHD for cleaning sooty surfaces is known per se.
In German Patent DE 100 57 862 C1, for instance, a method and an arrangement for reducing carbon-containing particle emissions from diesel engines is proposed in which the particles contained in the exhaust gas are deposited on filter surfaces, and the deposited particles are oxidized for the sake of regenerating the filter. The regeneration is done by means of nonthermal, electrical surface sliding discharges at the surfaces occupied by particles. In principle, the regeneration can be done either continuously or cyclically. In the case of a continuous regeneration, it is proposed that the requisite mean plasma capacity be regulated in accordance with the filter temperature, soot emissions, or the exhaust gas counterpressure, and in the case of a cyclical regeneration, by means of the exhaust gas counterpressure and the filter temperature. The goal of these concepts is to enable operating the plasma with as little expenditure of energy as possible. In particular for this purpose, it is provided that the filter ceramic be doped with catalytic materials.
Another method and an apparatus for plasma-supported decomposition of soot are known from German Patent DE 197 17 890 C1. Here as well, the filtered-out soot particles are made to react with oxygen on the principle of the dielectrically hindered discharge. In this case, at least one porous discharge electrode is especially provided in the flow course of the exhaust gas and is designed such that it is permeable to gaseous components but acts as a filter for the soot particles and traps them. The operation of the DHD can be done in a regulated way, and the voltage supply of the discharge electrodes can be switched on or off in response to the charge state of the electrodes with particles, which is ascertained by means of a sensor.
In the methods known from the prior art, the goal is to clean diesel exhaust gases per se, that is, to decompose soot particles that are present in exhaust gases. For this purpose, filter bodies are mounted in the flow course of the exhaust gas, where the soot particles are trapped and burned off by means of a DHD. It is therefore neither necessary nor contemplated, in the methods described until now, that the surface be kept in a defined sooty state.